A sexual act in which a male attempts to place his testicles into the asshole of his partner (preferably female but male-male action is possible also). Ideally, the asshole being to tight the testicles will repeatedly pop back out like a when you try to put a dog in a bath tub. The asshole should be well lubed and loose enough to allow partial testicle penetration. Not an ideal position for virgin or gaping assholes.

That girl is such a slut. Last night, she let me put the doggie in the bath tub!